The most important goal of experimental therapeutics for symptomatic Huntington's disease (HD) is to develop disease-modifying (neuroprotective) therapies able to slow progression. Rapidly advancing basic and translational research has identified numerous potential targets for neuroprotection and treatments based on them are creating an expanding pipeline for clinical trials. However, clinical trials are not close to keeping up with compounds for which there is already some rationale, as well as likelihood of safety and tolerability, and the gap is quickly widening. The development and use of biomarkers in clinical trials for HD have profound potential to increase the rate and accuracy with which compounds, and by implication their targets, can be assessed. Since HD is fundamentally a progressive neurodegenerative disease, neuro-imaging measures hold great promise as direct measures of disease progression that may also provide the necessary sensitivity and specificity to help assess disease modification in Phase II and Phase III clinical trials. During the current funding period, we used state-of-the-art neuro-imaging tools to show that regional brain atrophy in HD is early and progressive, and corresponds to clinical features, including the TFC. We also used these tools as a pharmacodynamic biomarker in a pilot study of high-dose Creatine, a leading candidate neuroprotective agent, in HD subjects. We found that high dose Creatine slowed brain atrophy, reduced measures of oxidative stress, and may have slowed cognitive decline. We are seeking to validate our neuro-imaging tools in a funded placebo-controlled, double-blind Phase III clinical trial of high-dose creatine as a biomarker of disease progression. Should Creatine slow progression and slow brain atrophy, it will be a crucial step in demonstrating the value of neuro-imaging as a pharmacodynamic biomarker of disease modification. PUBLIC HEALTH RELEVANCE: If Creatine slows the clinical progression of HD as measured by the TFC, we will learn how well neuro-imaging measures perform as pharmacodynamic biomarkers;it will set the stage for neuro-imaging as a useful indicator of potential neuroprotective efficacy in Phase II studies;it will be a first step towards eventually qualifying neuro-imaging as a potential secondary or surrogate endpoint in Phase III studies. Even without efficacy, we will learn how regional brain involvement underlies the complex symptomatology and course of HD.